The prior art describes several forms of interlocking pieces connected so that trucks and recreational vehicles can drive on to them and be leveled at an uneven site.
U.S. Pat. No. D426,933 shows one of a set of at least three interlocking blocks that are set up with two side by side and the third placed on top of halves of the bottom two to lock the three together. It has been found that this form of interlocking block suffers from substantial (over 20%) breakage in actual use with loaded trucks and recreational vehicles. The breakage is due to the particular strains placed on the blocks. A tire must be able to drive up a stepped ramp formed by the blocks and then come to rest preferably at the top of the third or top block or at any place on the stepped ramp. The strain that this makes on plastic pieces has been found to be unique in engineering of plastic parts.
U.S. Pat. No. 5,458,315 attempts to solve this breakage problem by using a heavily reinforced skeleton of brace plates. While in some ways preferable over the device of U.S. Pat. No. D426,933, the polygonal form of the raised part to interlock with a cavity in another block allows substantial lateral twisting that is not allowed with the square raised part shown in U.S. Pat. No. D426,933.
U.S. Pat. No. 7,040,603 shows that, for an interlocking type of ramp piece, from a floor plane level four square upward extensions are formed with four “key recesses” cavities formed therein. The underside of the each piece is formed to receive and interlock with the upward extensions.
There is a need for a device that would combine the non-rotation form of a leveling block with an improved non-breakable form.